1. Field of the Invention
The present invention relates to an oxidation catalyst useful for promoting the selective oxidation of carbon monoxide to carbon dioxide in the presence of hydrocarbons, and to a method of using the catalyst to selectively oxidize carbon monoxide in the presence of hydrocarbons.
It is often desired to reduce or eliminate the carbon monoxide content of gas streams containing hydrocarbons and carbon monoxide without significantly reducing the hydrocarbon content of the gas stream. The selective catalytic oxidation of carbon monoxide to carbon dioxide in the presence of hydrocarbons would be useful, for example, in removing carbon monoxide from process gas streams containing valuable hydrocarbons and in gauging the performance of catalysts used to treat exhaust or other waste gas streams, as described below.
2. Related Art
It is known in the art to utilize catalysts comprising platinum and bismuth components dispersed on zirconia-silica supports or titania-silica supports to catalyze the selective oxidation of carbon monoxide in the presence of sulfur compounds such as sulfur dioxide. These selective catalyst compositions are known to be useful to promote the oxidation of carbon monoxide while reducing or inhibiting the oxidation of sulfur dioxide to sulfur trioxide.
U.S. Pat. No. 5,157,204 to Brown et al, issued Oct. 20, 1992, discloses a platinum-containing catalyst for use in removing carbon monoxide and free oxygen from hydrocarbon-containing streams by contacting the stream with at least one platinum-containing catalyst composition at a reaction temperature in the range of about xe2x88x9230xc2x0 C. to about 200xc2x0 C. The catalyst compositions are said to consist essentially of platinum metal, iron oxide and an inorganic support including, among others, titania, alumina, zirconia and vanadia. Other catalyst compositions are said to consist essentially of platinum metal, palladium metal, at least one manganese compound and a tin-dioxide coated ceramic carrier. Another class of catalyst compositions is said to consist essentially of platinum metal, palladium metal, at least one manganese compound, at least one chromium compound and the tin-dioxide coated ceramic carrier.
U.S. Pat. No. 4,604,275 to Murib, issued Aug. 5, 1986, discloses a method for selectively oxidizing carbon monoxide in a hydrocarbon stream employing a supported catalyst containing cobalt oxide, the catalyst being prepared by impregnating an alumina support with an aqueous solution of a water-soluble alkaline compound whose anion is capable of forming a water-insoluble cobalt compound upon reaction with a water soluble cobalt compound.
In accordance with the present invention there is provided a selective catalytic material for selectively oxidizing carbon monoxide in a gas stream containing a hydrocarbon component in addition to the carbon monoxide. The catalytic material comprises a catalytically effective amount of rhodium, and a bismuth component present in an amount sufficient to inhibit the oxidation of the hydrocarbon component when the selective catalytic material is contacted under oxidizing conditions with such gas stream.
In one aspect of the present invention, the selective catalytic material is unsupported, for example, the rhodium and the bismuth component are combined in a co-precipitate obtained from a solution of a soluble bismuth salt and a soluble rhodium salt. In a related aspect of the present invention, the co-precipitate has been calcined in an oxidizing gas, e.g., air, at a temperature of at least about 750xc2x0 C.; for example, in a specific aspect the solution is an aqueous solution, the co-precipitate is obtained by evaporating the solution to dryness, and the co-precipitate is calcined at a temperature of from about 750xc2x0 C. to 950xc2x0 C.
In accordance with another aspect of the present invention, there is provided a supported selective catalytic material for selectively oxidizing carbon monoxide in a gas stream containing a hydrocarbon component in addition to the carbon monoxide. The supported catalytic material comprises the following components. A refractory inorganic oxide support has dispersed thereon a catalytically effective amount of rhodium, and a bismuth component. The bismuth component, which optionally may comprise Bi2O3, is dispersed on the support in an amount sufficient to inhibit the oxidation of the hydrocarbon component when the selective catalytic material is contacted under oxidizing conditions with such gas stream. Preferably, the rhodium and the bismuth component are dispersed on the same increment of support. While any suitable support may be used, a zirconia support is preferred.
Optionally, a catalytically effective amount of one or more platinum group metals other than rhodium, e.g., selected from the group consisting of one or more of platinum, palladium, iridium, ruthenium and osmium, preferably platinum, may be included in the selective catalytic material, in addition to the rhodium.
In accordance with one aspect of the present invention, the bismuth component and the rhodium are present in amounts that provide an atomic ratio of bismuth to rhodium (Bi:Rh) in the range of from about 0.5:1 to 4:1, for example, from about 1:1 to2.5:1.
In another aspect of the present invention, the bismuth component, calculated as elemental bismuth, comprises at least about 0.01 percent by weight, e.g., from about 0.01 to about 50 percent by weight, or from about 1 to about 30 percent by weight, of the supported catalytic material.
Another aspect of the present invention provides a method for selectively oxidizing carbon monoxide in a gas stream containing a hydrocarbon component in addition to the carbon monoxide, the method comprising the following steps. The gas stream is contacted under oxidizing conditions, for example, including a temperature of about 200xc2x0 C. or higher, with a catalytic material comprising a catalytically effective amount of rhodium and a bismuth component, for example, Bi2O3, present in an amount sufficient to inhibit the oxidation of the hydrocarbon component.
Another aspect of the invention provides for carrying out the above method by contacting the gas stream under oxidizing conditions with a selective catalytic material selected from one of the following. (1)The first catalytic material comprises a supported catalytic material comprising (a) a refractory inorganic oxide support, (b) a catalytically effective amount of rhodium dispersed on the support, and (c) a bismuth component dispersed on the support. (2) The second catalytic material comprises a co-precipitated catalytic material comprising (a) a catalytically effective amount of rhodium and (b) a bismuth component and obtained by co-precipitation from a solution of a soluble bismuth salt and a soluble rhodium salt; and wherein the bismuth component is present in an amount sufficient to inhibit the oxidation of the hydrocarbon component under the oxidizing conditions.
Other method aspects of the present invention provide for contacting the gas stream under oxidizing conditions with any one of the selective catalytic materials as described above.
Other aspects of the present invention will be apparent from the following detailed description of the invention and specific embodiments thereof.
As used herein and in the claims, the term xe2x80x9coxidizing conditionsxe2x80x9d means that the gas stream contains at least sufficient oxidant, e.g., oxygen, to oxidize the CO content of the gas stream and is at a temperature which is high enough for the catalyzed carbon monoxide oxidation reaction to take place. The defined term also includes conditions where excess oxygen over the stoichiometric amount required to oxidize all the carbon monoxide is present, and further includes conditions where excess oxygen over the stoichiometric amount required to oxidize all the carbon monoxide plus hydrocarbons in the gas stream is present.
As used herein and in the claims, the term xe2x80x9cplatinum group metalsxe2x80x9d means and includes platinum, palladium, rhodium, iridium, ruthenium and osmium.
As used herein and in the claims, the term xe2x80x9cstabilizedxe2x80x9d used with reference to a refractory inorganic oxide support, means that the support has been stabilized against thermal and hydrothermal degradation by incorporation therein of another refractory metal oxide which tends to stabilize the lattice of the support. For example, it is well known in the art to thermally stabilize alumina or zirconia supports by incorporation therein of one or more stabilizing metal oxides such as ceria, lanthana or other rare earth metal oxides. To cite one example, U.S. Pat. No. 4,171,288, issued to C. D. Keith et al, discloses the stabilization of alumina support by the use of stabilizers such as zirconia, titania, alkaline earth metal oxides, such as baria, calcia or strontia or, more usually, rare earth metal oxides, for example, oxides of cerium, lanthanum, neodymium, praseodymium and mixtures of two or more thereof, including the commercially available mixtures of rare earth metal oxides. It is also known to thermally stabilize bulk ceria support with alumina, as disclosed in U.S. Pat. No. 4,714,694 of C. Z. Wan et al. Generally, the stabilizing material is impregnated into the particulate refractory inorganic oxide support via a solution of a soluble salt of the metal of the stabilizing compound. The impregnated support is then dried and calcined to convert the soluble salt, e.g., a nitrate, to the oxide.
As used herein and in the claims, the term xe2x80x9chydrocarbonxe2x80x9d or xe2x80x9chydrocarbon componentxe2x80x9d shall mean and include any hydrocarbons which are susceptible to oxidation by being contacted with an oxidation catalyst under oxidizing conditions. The term includes alkanes, paraffins, alkenes, alkynes, olefins, aromatic compounds and mixtures of two or more thereof.
As used herein and in the claims, a xe2x80x9csupportedxe2x80x9d catalytic material of the present invention is one in which the bismuth and a rhodium components (and any optional metal components) are dispersed on fine particles of a refractory inorganic oxide support. As used herein and in the claims, an xe2x80x9cunsupportedxe2x80x9d catalytic material of the present invention is one in which the bismuth and rhodium components (and any optional metal components) are present as fine particles of the components themselves, such as particles obtained by precipitation of the components from solutions or complexes of salts of the metals of the components.